Weakening barriers and novel communities expected for North American boreal birds

Submitted by editor on 13 September 2016. Get the paper!
The Cape May Warbler (Setophaga tigrina), a spruce-budworm specialist that breeds regularly throughout boreal coniferous forests of North America east of the Western Cordillera, is one of 31 species considered likely to expand its range into the Alaskan boreal interior by 2040. Evidence of breeding activity has recently been recorded in south-central Alaska (Gibson and Withrow 2015). Photo by Jacob Spendelow  (http://www.tringa.org).


By Diana Stralberg

What are the limits to range expansion under climate change? Bioclimatic niche models are often used to map shifts in suitable climate-space for a species and to estimate potential future distributions. But many species are already absent from portions of their climatically suitable niche, presumably due to physical or behavioral barriers to dispersal. Indeed, approximately half of the North American boreal songbird avifauna is currently absent from the Alaskan interior boreal region, which is separated from Canadian boreal forests by the mountain ranges of the northwestern cordillera. What do these species have in common? And what are the implications under climate change, when climatic connectivity between Canada and Alaska is projected to increase?

The Tennessee Warbler (Oreothlypis peregrina), one of the most abundant breeding birds of boreal forests in eastern Canada, is another species considered most likely to become a regular breeder in the Alaskan boreal interior by 2040. Evidence of breeding activity has recently been recorded in both interior and south-central Alaska (Gibson and Withrow 2015). Photo by Jacob Spendelow  (http://www.tringa.org).

We addressed these questions by analyzing the relative importance of life history, competition, and climate history in explaining the occurrence versus absence of 80 species of songbirds in boreal Alaska during the breeding season. In addition to quantifying current factors, we conducted paleo-hindcasting of each species’ distribution during the last glacial maximum ~ 20 000 years ago and also during the warmest period of the Holocene, ~ 6000 years ago. Accounting for species’ relatedness and for climatic suitability in Alaska, we evaluated multiple candidate models to predict species’ occurrence there. We found that connectivity of climatically suitable habitat across the northwestern cordillera—currently and during the mid-Holocene period—were the strongest predictors of current Alaska presence. Migratory status and locations of ice-age refugia were also important, but not necessarily independent of phylogeny. We found no evidence of physical traits or competition playing a role.

Scenic view of interior boreal forest with mountains in the background, covered with fresh September snow, along the Parks Highway, Alaska. Photo credit: Donna Dewhurst.

Applying our best-supported model to future conditions of climatically suitable habitat across the northwestern cordillera, we found that nearly all of the 42 boreal songbird species not currently breeding in boreal Alaska are likely to expand their ranges into the region by the end of the century (most within the next 30 years). This prediction is supported by the recent increase in numbers of sightings of new species within the Alaskan boreal region, all of which are among the most likely “invaders” based on our models. This suggests that the perceived barrier of the northwestern cordillera may be easily overcome as climate change improves conditions for many forest species across this region, and raises numerous issues regarding future species management and the integrity of current ecological communities. As species begin to trickle into new regions at different rates, novel communities will increase and are likely to become the norm. Conservationists and land managers may need to reconsider conservation policies and strategies in light of evolving ecological communities. 



Gibson, D. D. and Withrow, J. J. 2015. Inventory of the species and subspecies of Alaska birds, second edition. — Western Birds 46: 94-185.